It's a hobby for me but I have made some money at it. I have two aquarium related patents, one is the "Majano Wand" and another is a seahorse feeder. The Majano Wand is sold by Marine Depot. I don't own the company and don't want, anything to do with the company. I just get royalties as I didn't want the headaches but I still hold the patent.
The seahorse feeder used to be sold by "Brine Shrimp Direct" and after making thousands of them, I got tired and stopped. But they made some money.
I also made and shipped a few thousand of these to Marine Depot until I got tired of building them and my acrylic tube manufacturer moved to China.
I wrote a book that is doing fairly well. The "Avant -Garde Marine Aquarist" 100% of the profits from that go to Multiple Sclerosis research in my wife's name so I really can't count that as profit.